1. Field of the Invention
The present invention relates generally to microwave integrated circuitry and more particularly pertains to MIC (microwave integrated circuit) assemblies having a continuous ground plane for one or more MIC's.
2. Description of the Prior Art
In the field of microwave integrated circuitry, it is the practice to employ substrates of a ceramic material on which are disposed active and passive microwave circuit elements and devices. The lower sides of substrates are metallized to provide a ground plane for the devices and the remaining portions of the circuit elements are disposed on the upper sides of the substrates.
Terms such as upper, lower, top, bottom, etc., are used herein for convenience of description only and imply no required or preferred orientation for the object described.
Individual MIC's are incorporated into an assembly by mounting one or more of them on a structure which serves as a support member or carrier for the MIC's. Interconnections are made between the MIC's. Interconnections are also made to external connectors which are typically coaxial connectors having their outer housings fastened to, and grounded to, the carrier.
Where the carrier is of sheet metal or metal plate, it is usually referred to as a chassis. A chassis is typically fabricated from a single piece of conductive metal and therefore exhibits good electrical and thermal conduction throughout. Such a single piece of conductive material is described herein as being capable of serving as a structurally continuous ground plane.
In the fabrication of prior art MIC assemblies, it has been regarded as essential to provide good conductive electrical contact between a primary ground plane and ground plane metallization on the lower side of each MIC substrate. The primary ground plane is typically a structurally continuous ground plane such as is provided by a chassis. Where ground plane metallizations on substrates are present and conductively connected to a primary ground plane provided by a carrier, such ground planes are referred to as discontinuous in this specification. Good electrical contact between a chassis and substrate metallization tends to assure that the performance of the MIC will conform to that which was intended by reason of its design or to that which was observed in tests of the MIC as an isolated unit or, ideally, both of these.
However, what suffices as good electrical contact may vary as the frequency of operation varies. Parasitic elements introduced into a circuit by reason of less than ideal grounding may have a negligible effect on circuit performance when the circuit is operating at, for example, 500 megahertz but may cause serious degradation of circuit performance at, for example, 10 gigahertz.
It will be apparent to those skilled in the art that good electrical contact between substrate metallization and a chassis surface cannot ordinarily be obtained by simply placing the metallized surface of the substrate on the chassis. Parts fabricated to reasonable specifications for most applications are neither optically flat nor highly polished. The percentage of substrate metallization in actual metal-to-metal contact with the chassis would tend to be small due to random curvatures in the parts and roughness or irregularities on their surfaces.
Several different techniques are used in the prior art to provide an improved connection between the ground plane metallization of an MIC substrate and a chassis. These include, for example, the use of mechanical fasteners such as screws, conductive adhesives such as the epoxies filled with powdered conductive metal, and reflowed solder.
Screws are sometimes used to apply pressure at the interface between the substrate metallization and the adjacent surface of the chassis. This increases the area of metal-to-metal contact but typically only in the neighborhood of a screw. Care must be exercised in using a mechanical fastener mounting technique such as this since excessive pressure will fracture the dielectric substrate which is typically of alumina or beryllia.
Screws have been used successfully for temporarily mounting MIC's in test fixtures for preliminary checkout. Test results have been found to be reasonably reproducible and in reasonable conformance to design expectations up to about eight gigahertz. However, difficulties have been encountered at higher frequencies. At these higher frequencies, screw-mounted MIC circuitry can exhibit substantial variation in its behavior from design expectations. Experience has shown that MIC performance is especially sensitive to the quality of the direct contact between the lower surface metallization and the chassis at certain places. One such place, for example, is at or near the edge of an MIC substrate directly beneath microstrip interconnections between the MIC and another MIC or a coaxial connector. A technique used in the laboratory to improve the electrical contact is to force pieces of gold ribbon between the substrates and chassis at these places.
The metal-filled epoxies can be used to good advantage to provide a strong attachment having both good thermal contact and good electrical contact between MIC ground plane metallization and a chassis. This is so because the epoxy, before curing, can be caused to fill the interstitial space between opposing surfaces thereby removing the effects of surface curvature and irregularities. However, the use of expoxy has a disadvantage in that, once cured, its attachment is typically permanent. If one individual MIC then fails, an entire assembly may have to be discarded. However, see U.S. patent application Ser. No. 821,034, filed Aug. 1, 1977, by the applicant herein and assigned to the assignee of this application; that application describes a releasable mounting for MIC's employing both mechanical fasteners and metal-filled epoxies.
In addition, the use of metal-filled epoxy adhesives for mounting MIC's in assemblies becomes seriously disadvantageous at frequencies higher than about one gigahertz because the electrical loss in the adhesive then becomes excessive.
Several problems have been encountered in the use of solder for attaching MIC's to a chassis. As one example, the choice of a flux is a compromise. The fluxes which are active enough to be effective are detrimental to microwave devices. Those fluxes which are benign to the devices produce wetting of the surfaces no better than about 20% to 30% in some observed processes. Thus, the presence of a good ground plane connection to the chassis directly beneath microstrip interconnections between MIC's and between an MIC and a coaxial connector center conductor, cannot be assured. As another example, solder often flows up between two closely-spaced MIC substrates during the reflow soldering process. If the upward reach of this solder is sufficient, short-circuiting of an upper surface element may occur while a lesser reach of solder may still cause detuning by introducing parasitic capacitances.